Coming in April 2018, Mollom a service we love to suggest to stop spam on Drupal based websites who have forms will be shutting down. Currently with about 37,000 websites using the platform there's a need for alternatives. Here's our suggestions:
1. Use Google's ReCaptcha service: https://www.drupal.org/project/recaptcha
2. Use the Honeypot module: https://www.drupal.org/project/honeypot
3. Use the antibot module: https://www.drupal.org/project/antibot
Each of these has their pros and cons, we tend to use the ReCaptcha module since it's backed by Google and the updated versions have a one click "not a robot" check that makes it easier for all of our customers end users to pass the spam filter checks with minimal effort. Honeypot is a great module as it uses a hidden field that normal visitors can't see, but bots tend to fill out. If filled out it's noted as spam.